We have developed several methods to study platelet activation ex-vivo and in-vitro. These studies were made possible by the development and implementation of techniques to isolate platelets from human blood without inducing platelet activation or secretion. We have developed nine (9) monoclonal antibodies which react only with activated platelets. Our results have shown that in some forms of von Willebrand's disease, platelet activation is present and causes thrombocytopenia. In patients with arteriosclerotic heart disease, after exercise platelet activation is increased. This suggests that some degree of platelet activation occurs in these patients following strenuous exercise. In volunteers receiving endotoxin, spontaneous platelet activation or enhanced agonist-induced platelet activation is not observed. Exercise in individuals with minor or mild atherosclerosis does not induce platelet activation.